OAK RIDGE, Tenn. (AP) — Officials like to refer to the Y-12 National Security Complex as the Fort Knox for highly enriched uranium, which is why an unprecedented incursion by an 82-year-old nun and two fellow protesters has critics mocking the notion that the weapons plant is secure.

Operations resumed last week after being shut down over the embarrassing incident 18 days earlier. The Department of Energy has called on the contractor that runs the sensitive facility just west of Knoxville to explain why it shouldn't be replaced.

Y-12 makes uranium parts for every warhead in the U.S. nuclear arsenal, dismantles old weapons and is the nation's primary storehouse for bomb-grade uranium.

Officials insist that despite the more than two hours that the protesters went unchallenged on the facility, there was never any danger of them getting to materials that could be detonated on site or used to assemble a dirty bomb.

The Highly Enriched Uranium Materials Facility, or HEUMF, is designed to withstand earthquakes up to a 7.7 magnitude, tornado-force winds of up to 200 mph or the impact of a general aviation aircraft. It could also withstand a ground attack, officials said.

"Our (protection force) is deployed so that any serious attempt to attack the facility would be repulsed well in advance of any credible threat," Steven Wyatt, a spokesman for the National Nuclear Security Administration at Y-12, said in an email.

But Peter Stockton, a former DOE adviser on nuclear security in the Clinton administration and a senior investigator with the Project On Government Oversight, said the incident hasn't been taken seriously enough because the intruders had no violent intentions.

"We were lucky in that regard that it was the nun and her cohorts, rather than a serious terrorist outfit," Stockton said. POGO, a Washington-based independent watchdog known for exposing overpriced military parts and other government shortcomings, has been a frequent critic of security lapses at the facility.

Stockton called the July 28 intrusion the "only serious penetration of a plant" that he's aware of since becoming involved in nuclear security issues in the mid-1970s.

"It is simply (expletive) unbelievable," he said.

Other than striking out in the pre-dawn darkness, the three protesters did little to conceal their nearly half-mile trek into the restricted area where signs warn intruders they could be shot.

According to court documents, they used bolt cutters to get through three fences, tripping alarms in the process. They told acquaintances in the peace movement that they spent more than two hours inside the restricted area. The action culminated in the protesters spray-painting and throwing blood on the walls of the white fortress-like HEUMF structure.

"Despite receiving numerous alarms from the multi-layered sensor system in the fence line, the protective force failed to react to the protesters as they cut through the three fences," according to letter.

And once they did decide to investigate, responders didn't know what to do with the protesters until a supervisor took control of the situation.

Wyatt, the Y-12 spokesman, said after the plant ended its stand-down that the main security force operated by WSI Oak Ridge, formerly Wackenhut, has been downgraded to a subcontractor in response to the incident. Other improvements also were made, but he declined to provide specifics because the information is considered sensitive.

After the breach, the president and general manager of the Babcock & Wilcox division that runs Y-12, Darrel Kohlhorst, retired from his job. He told The Knoxville News Sentinel that the company would emerge stronger because of the incident.

"Well, I think it did show us we had some weaknesses. We had some deficiencies," Kohlhorst told the newspaper. "The team has really attacked those things and corrected them, and I think we're actually going to be a lot stronger coming out of this thing."

Stockton said the muddled response on the night of the intrusion raises questions about what might have happened under a different set of circumstances.

"You get through the fences, you get to the building, and if you have special forces guys — dedicated guys who are suicidal and heavily armed — all you do is blow the door off or blow a hole in the side of the building," he said.

Others questioned whether the protesters got close enough to do any real harm.

The intruders never came within reach of weapons-grade materials, U.S. Sen. Bob Corker, R-Tenn., said in a phone interview Wednesday after a tour of the facility.

"There were still lots of other levels they would have had to go through, but again, you can't tolerate this," he said. "I do think what it's done is that it's served as a wake-up call."

Matthew Bunn, an associate professor at Harvard University's John F. Kennedy School of Government, said his own tour of the facility leads him to agree that penetrating the interior of the building would be a difficult proposition.

Y-12 has always been a secretive place. The government tucked it and other bomb-making facilities into a valley about 30 miles west of Knoxville during World War II and built a guarded city around it. The city of Oak Ridge is open today, but most of the Y-12 facilities can't be seen from the road and visitors must get a security clearance to enter.

One indicator of how serious authorities consider the July breach is how they're treating the arrested trio.

Anti-war protesters have rallied at the gates of Y-12 for decades around the anniversary of the bombing of Hiroshima. Some deliberately trespass or block traffic to provoke arrest and call more attention to their cause. Some years, authorities have tried to deprive them of the notoriety by refusing to prosecute. Sometimes they go to federal court, but the stiffest sentence ever meted out was less than a year in prison.

This time, federal prosecutors have thrown the book at the three protesters, charging them with offenses that could carry cumulative prison sentences of 16 years for Sister Megan Rice of Las Vegas, Michael Walli of Washington and Greg Boertje-Obed of Duluth, Minn.

"That's the reaction to the embarrassment," said Ralph Hutchison, of the loose-knit Oak Ridge Environmental Peace Alliance.

Previous protests around the plant — including one less than three months after 9/11 — have led to millions of dollars of security upgrades. But those haven't prevented repeated lapses.

"We're paying all that money for an illusion of security — and you really can't secure the plant," Hutchison said.

On New Year's Day 2002, a dozen protesters marched onto the Y-12 facility carrying 14-foot flags and lighted candles. They stopped three times for prayer services, but were arrested only after being spotted by a passing motorist who reported them to guards.

Since then, more guards have been added, concrete barriers have been built and other security measures taken to meet protective standards described as three times tougher than before 9/11.

But security miscues have persisted.

In 2003, guards were accused of cheating on a mock assault drill, though security contractor Wackenhut Services Inc. denied it. Later, a refrigerator was shot with a live round that had become mixed with blanks at the guard training center.

Confusion during another training exercise in 2004 allegedly put drill participants at risk of being shot by a "shadow force" of on-duty officers.

Seven guards were caught napping at Y-12 between 2000 and 2008, and Bunn said the security lapses show the inherent difficulty in maintaining perpetual vigilance.

"The reality is nobody ever attacks these facilities," he said. "And getting everyone constantly on their tippy-toes of preparedness for an attack that never comes is a difficult problem."

The most recent protest was in response to the preparations for building of a new multibillion-dollar uranium processing plant at Y-12, said Hutchison, of the anti-war group. The intrusion should cause officials to "recognize that building the new bomb plant carries with it significant security risks."

Hutchison said the security breach has security teams at the plant on their toes — at least for now.

"I doubt anybody's relaxed and casual now," he said. "But give it six months or a year-and-a-half, and then what?"